Selective system



March 16, 1943. H. M. BACH SELECTIVE SYSTEM Filed Aug. 1940 7 Sheets-Sheet 2 F/Gia.

F/G-4. E.

FIG. 5.

v INVENTOR BY Hf/VRY M. 8409 Z! fa ATTORNEY M ch16, 1943. H. M. BACH i SELECTIVE SYSTEM 7 Sheets-Sheet 7 Filed Aug. 8, 1940 NNN NNN

INVENTOR HE/VR) N. EACH id i ATTORNEY ?atented Mar. 16, 1943 4 UNITED STATES PATENT OFFICE Sm SYSTEM 1 Henry M. Bach, Woodmere, N. Y., minor, by

mesne assignments, to Patents I York, N. Y., incorporation of New poration, New York Research Cor- Application August 8,1940, Serial No. ans-11 11 Claims.

The present invention relates to electric wave translation systems and methods of operating same, more particularly to a wave filter for translating modulated carrier signal energy.

There frequently arises the problem in practice in electric wave translation systemsor filters for transmitting modulated signal energy to amplify a discrete band of frequencies and o elimi-- nate all other frequencies. Specifically, in radio or similar receivers for modulated signal energy it is desirable to amplify a band of frequencies including the carrier and the side band frequencies characteristic of the signal or message being transmitted and to completely eliminate all frequencies outside the highest side band-frequency desired to be received in order to minimize or suppress the reception of interfering signals. g

The above obiect'is usually accomplished We system of cascaded resonant circuits designed to offer high impedance to the desired frequency band and to oifer a substantially lower impedance to frequencies outsidepthe desired fre- 1 quency band.

The known systems used at the present have the disadvantage of requiring a large number of cascaded amplifier stages to yield anything like sharp attenuation of the undesired frequencies.

' justed easily to have any desired shape to suit A further disadvantage of the known systems is the fact that they employ band-pass filters with greater than optimum coupling and accordingly the timing of one ofthe elements of the filter reacts upon the other elements in such a manner is to make proper alignment and tuning extreme- There is also the problem of obtaining extreme selectivity in a communication type receiver. Heretofore, this has rquiredthe employment of pecially inthe design of superheterodyne receiv era is to substantially eliminate the second channel or image frequency interference from the first detector or converter stage without the use 'of an undue number of radio frequency pre-selector stages or without the use of complicated band-pass circuitspreceding the converter stage.

Accordingly, it is an object of the present invention 'to provide new and novel means for accomplishing the above results in a simple manner, thereby removing the'need for an increased number of cascaded amplifier stages with their attendant difficulties and providing an electric wave translation .system or filter having a frequency response characteristic which may be adexisting design requirements.

Another object is to provide a wave filter having a minimum of inter-action between the tuned circuits forming the elements of the filter.

.A further object is to provide means for obtaining extreme selectivity in a radio or equivalent recelverby utilizing a minimum of tubes and parts and for providing a high degree alyzer adapted to 'allow the determination and accurate measurementof the fundamental irequency and the various harmonic components of a complex electric wave.

a great number of cascaded amplifying stages.

0n the other hand,- the number of cascaded stages thatmay be used is limited ,in practical design due to inherent tube noise, self-oscillation. physical space considerations and various other requirements. Moreover, the use of a num-v her ofsharply tuned cascaded'stages tends to sharpen the entire resonance curve rather than Another object is to provide a simple and effective means for rejecting the image-frequency in a superheterodyne receiver.

These and further objects and aspects of the V invention will become more apparent as the folto eliminate the outer portions or skirts without cutting the peak. In other words, these methods are not adapted to approach the ideal of a flat top curve with sharp cut-offs desirable in practice. As a result of this drawback the desired side bands are reduced or suppressed when extreme selectivity is duh-ed resulting in signal distorlowing detailed description proceeds taken with reference to the accompanying drawings forming part of this specification and wherein;

Figure 1 is a basic circuit diagramflof a selective filter-or translationsystem embodying the principles of-the invention, .Figure2isadiagr'amofaradiofrequency amplifier-and detector embodying a selective fllter. for varying the frequency response charac- Y teristic of the detected signal in accordanceiwi the invention,

Figurellisamodlfieddlagramofaradiofreamthemobiunmetwflhmmflcaesquemyamflmermddetecwrforrecemngaminvention,

plitude modulated carrier signals and embodying a selective filter according to the invention for varying the frequency response characteristic,

Figures 4A to 4E are explanatory graphs illustrating the variation of the frequency response curves of an amplifier shown in Figures 2 and 3, Figure 5 shows the variation of the detected audio frequency wave obtained from Figure 2,

Figures 6A to 6E show the variation of the wave shape of the audio frequency output current or voltage of Figure 2 when using a high-Q resonant impedance in the filter circuit according to the invention such as a piezo-electric crystal as shown in Figure 9, a Figures '7 to 10 illustrate further modifications of basic circuits embodying the principle of the invention,

Figures 11A to 11F show further resonance or frequency response curves which may be obtained from systems constructed in accordance with the Figure 12 is a vector diagram explanatory of the function of the invention,

Figures 13 to 17 show still further modifications of basic circuits embodying the principle of the invention,

Figure 18 is a diagram showing the radio frequency spectrum of a modulated wave including distortion components,

Figure 19 shows apractical embodiment of the invention for analyzing the distortion components contained in an electrical wave,

Figure 20 shows the variation in plate current in the circuit of Figure 19 as a function of frequency,

Figure 21 shows a complete radio receiver embodying a band-pass system of the type according to the invention, v V

Figure 22 illustrates schematically a further embodiment of the invention for multiplex communication through a single signalling channel, and

Figure 23 illustrates the employment of the invention in a superheterodyne receiver to effect rejection of the image frequency signal.

Like reference characters identify like parts throughout the different views of the drawings. Referring more particularly to Figure l, ther is shown a simple circuit constructed in accordance with the principle of the invention. Item is an electron discharge tube in the example shown a. tube of the hexode type comprising a cathode H, a firstinput or signal control grid I2,

a further control grid |3,-a screen grid l4 located between the control grids l2 and I3 and an anode or plate Hi. If in a tube of this type the control grids l2 and i3 are suitably negatively biased .with respect to the cathode H, such as by the provision of a biasing network l1, comprising a resistance and a shunt and by-pass condenser placed in the cathode-to-ground lead or by any other suitable biasing arrangement, and if suitable positive potentials are applied to the screen grid M and to the'plate Ii with respect to the cathode, electrons emitted from'the cathode II will be accelerated through the .meshes of the first control grid l2 towards the positive or screen grid M. Most of .the electrons will flow through the relatively wide. meshes of thescreen grid and form a concentrated space charge or virtual.

cathode in the region between the screen grid l4 and the second control grid IS. The magnitude of this space charge or virtualcathode and its distance from the control grid I8 is a function ofthe magnitude of the positive potential on the 75 grid Hi, the negative potentials on the control grids l2 and I3, and of the intensity of the electron space current emitted from the cathode II.

If a radio frequency signal is impressed between the input grid I2 and the cathode II which signal may be supplied from any suitable source. preceding the input terminals a--b, a voltage will be set up on grid l2 causing the electron stream flowing from the cathode H to the virtual cathode to be varied or modulated, that is, the charge density of the virtual cathode adjacent to the control grid 13 will fluctuate at the frequency of the input signal. Item 20 is an impedance element having a value Zr connected between the grid l3 and ground. If this impedance Z: is of purely resistive character, it will be found. that a displacement current will be induced in the circuit of grid I3 flowing from this grid through the resistance to the cathode. This current is of the same frequency as the input signal but phase shifted by90" with regard to the input potential on the grid l2. In other words, the current from the grid 63 through the resistive impedance Z: to ground will be phase shifted by- 90 with 25 respect to the signal voltage between the grid l2 and cathode II. If the impedance element 20 between the grid 13 and ground or cathode has the form of a parallel resonant circuit and the resonant frequency of this parallel resonant cir- 39 cult is varied about the frequency of the incoming signal the following phenomena will be observed.

Whenever the resonant frequency of impedance 2b is equal to the frequency of the incoming signal impressed by way of terminals ab, Zr will ofier pure ohmic resistance to the quadrature potential induced on the grid 63 and accordingly the voltage across grid 53 and ground or cathode will be in phase with the current through this circuit and the potential established on the grid 40 I3 will therefore be phase shifted by 90 with respect to the potential on the input grid l2. If the impedance element 20 is tuned to a frequency slightly higher or slightly lower than the frequency of the incoming signal, Z: will offer capacitative or inductive reactance to the displacement current depending on the sense of departure of the resonant frequency of impedance 20 from the frequency of the incoming signal. Accordingly, therefore, the voltage developed between the grid l3 and cathode or ground will either lagthe current through the circuit by 90 or lead the current by 90 dependingupon the sense and magnitude of the relative frequency departure between the resonant frequency of 20 and the impressed signal frequency. Consequently the resultant voltage developed on the grid i3 will be either in phase with the voltage on the grid l2 or 180 out of phase with the latter. By suitably tuning impedance 28 to either the signal frequencyor a frequency above or below the frequency of ,theinput signal any intermediate degree of phase shift between the .values 0 and 180 may be obtained for the potential established on grid IS with respect to the potential on grid I2.

An analysis of the grid current in a vacuum tube ofthe type shown comprising a pair of control grids placed in the path of the electron space current andhaving potentials of the same frequency but varying relative phase angle impressed upon said grids shows that the plate output current may be represented substantially by the following formula:

. i=a1eik1E"+azezE+a1aie1ez rovided higher order terms may be neglected and el and e: representing the alternating voltages impressed upon or established on the con trol grids. The first two terms of the above formula will cause avectorial resultant current in the output circuit which will develop a potential varying independence upon the resultant phase angle between the grid voltages er and ea. From this it will be seen that by suitablyituning the resonant impedance means 20, any desired fre-e quencyor band of frequencies may be substan tially rejected or accentuated in the output circult to obtain a desired response characteristic as will be further explained with reference to the vector diagram shown in Figure 18.

. Item H3 in Figure 1 is a by-pass condenser connected between the screen grid I4 and ground in a manner well known. The output circuit of the tube may contain any suitable coupling impedance Z designated by numeral 2| to develop out-' put voltage between terminals c-d. Item 22 is a' V by-pass' or smoothing corfidenser to eliminate fluctuations of the plate-voltage in a manner well understood. Impedance Il may be' designed to respond to either audio or radio drequeney variations of theplate current. For example it may be comprised of an ohmic resistance shunted by g a radio frequency by-pass condenser as shown in Figures 2, '7 to 9, 13, 16 and 1'1, said by-pass condenser offering little or no impedance to the radio frequency components and offering high impedance to the modulating signal such as audio frequency signal components of the plate current. Alternatively, the impedance 2| may be a low-pass filter as ,shown in Figure 19 or a radio frequency impedance as shown in Figures 3,

l0, l4, 15, 21 and 23, as will be further described. hereafter.

The system as described is especially suited for modifying the response characteristic of a translation circuit for an amplitude modulated carrier signal e comprising a carrier component and upper and lower side, bands as represented by the following general formula well known to those skilled in the.art:

s 2 mm grids l2 and II will vary in phase and amplitude depending on the tuning adjustment of the resonant impedance means 20 connected between the control grid 13 and cathode or ground. Accordingly, inthe output circuit either one of the side bands cos a c-1.);

or I cos 2 0w? or one side band and the carrier Eo sin 21, or may be accentuated or attenu-- both side bands ated depending on the tuning adjustment of the resonant impedance". This applies to w'single side band frequency considered in the above Accordingly, therefore, the output frequency developed across the impedance 2! will either atv tenuateor accentuate within limited frequency rangesthe component audio or=other modulating signal wave and the response curve of the system will be of the type as shown in Figures 4A to 4E to be explained hereafter.

If impedance 2!! is a radio frequency impedance such as a parallel resonant circuit tuned to the frequency the input signal, there will be developed a radio frequency potential across the output terminals c-d comprising a band of radio frequencies (side bands) corresponding to the modulating frequency wave and the carrier and the side bands ofthe input signal will be amplified differentially in dependence upon the tuning adjustment for the resonant impedance 2., in a manner as is readily understood from the above. Thus, one side band and the carrier may be attenuated or accentuated, or the carr eralone t may be accentuated or attenuated or finally, a single side band may be modified in a. similar manner depending on the relative detuning of the resonant impedance 20 with respect to the input signal frequency.

Referring to Figure stage radio frequency amplifier feeding a selective detector stage constructed in accordance with the invention to modify the frequency response characteristic of the system. The radio frequency amplifier stages are of standard design comprising a pair of pentode vacuumtubes 24 and 25, a tuned inter-stage'ccupling transformer 26 and a tuned output transformer 21. The secondary vvoltage supplied by the latter is impressed between thegrid l2 and cathode li of a detector tube to of substantiallythe same type as shown in 1, embodying a parallel tuned circuit comprised of a variable condenser 30 and an inductance 3i forming a resonant impedance means connected between the control grid l3 and ground or cathode. The circuit til-M is suitably isolated or magnetically shielded from the remaining parts of the system such as by the aid of a grounded metallic screen indicated at 32. The tube is biased to operate as a plate detector and includes a low-pass coupling network in the plate circuit comprising a series resistance 34 and a pair of by-pass condensers adapted to develop an audioifrequency potential impressed by way of coupling condenser 33 upon the vertical deflecting plates "and )1 of a cathode ray oscilloscope connected between the output termihalsc-d,

'flecting plates 40 and ll.

said oscilloscope having a pair ofhorizontal de- The positive potentiai' applied to the screen grid H of-thedetector. Wis

reference to Figure l. The tuned circuit Iii-3| comprising an inductance. and a condenser in parallel and offering an impedance 2:, Figure 1, is resonant to a frequencyclose to the frequency of the incoming signal impressed by way of terminals 0-41 and accordingly a phase shifted potential of the same frequency as the incoming signal 2, .there is shown a two-- and sense of the phase shift being proportional to the departure of the resonant frequency of the circuit 30-31 from the incoming signal frequency. Accordingly, therefore, in the output circuit of tube ii there will be a variation in the voltage developed across the resistor 34. The shunt condensers for the latter will by-pass all radio frequencies from the plate circuit and thus only audio frequency variations will be developed across the resistor 34. The overall resonance curve of the amplifier may be substantially controlled by suitably adjusting the circuit 30-3! to a resonant frequency at or near the frequency of the incoming signal and the output current or potential may be visibly observed on the screen of the cathode ray oscillograph 35 as a function of the setting of the variable condenser'30 of the resonant impedance-in. a manner to be further described hereafter.

If an unmodulated radio frequency input signal is varied about its normal frequency, resonance or frequency response curves as shown inFigures 4A to 4E may be obtained, the different response curves corresponding to different settings of the condenser 30. For example, if the input frequency is varied very rapidly such as by elec-'- tronically controlled reactance means there will be visibly traced on the screen of the oscilloscope resonance curves of the type shown on Figures 4A to 4E provided a sweep voltage equal to said frequency variation is impressed upon the horizontal plates 60, M of the oscilloscope.

Figure shows the variation of the output voltage 12 as a function of the setting 1n, n, o, p and q of the tuning condenser 30 obtained on the'screen of the oscilloscope'from a system of Figure 2 or Figure 3 when an amplitude modulated input signal is held at a constant frequency and a suitable time base voltage is applied to the horizontal deflecting plates of the oscilloscope.

Figure 2 shows a circuit embodying the principle of the invention in the form of a detector for modulated radio waves to eifect a variable frequency response in the detected output current.

Figure 3 shows a radio frequency amplifier and detector of the same type as shown in Figure 2 with the exception that the frequency response control tube Ill is used as a radio frequency amplifier rather than as a detector and the radio frequency output signal is detected separately in a known manner by a diode detector 45. The frequency response is controlled in the same manner as in Figure 2 by adjusting the tuning frequency'of the resonant impedance means, in the example shown the parallel tuned circuit 3ll 3l connected between the virtual cathodeexcited control grid l3 and cathode or ground in the manner proposedby the invention. Items 42 and 43 represent the pre-amplifying stages shown schematically. Item 44 is the resonant outputeo transformer for the tube 10 energizing the rectifying circuit comprising diode 45 and a load resistance 48 shunted by a condenser 41. The rectified potential developed by the resistance 44 is impressed between the vertical deflecting plates similar to that shown in Figure 2.

- 36 and 31 of the oscilloscope 35 in a manner Referring again to Figure 2. if the tuned resonant circuit 30-" is replaced-by a, resonant irnpedance having a substantially higher Qsuch as a quartz crystal shunted by a high ohmic impedance to provide a' direct current return path for the control grid l3 suchas shown in detail in Figure 9, it is found that the selectivity curve of the system will be greatly sharpened. If a 2,313,011 will be developed upon the grid ii, the magnitude radio frequency carrier I modulated accordin to a frequency f: in the manner described before l shown in Figure 18 which side bands are removed from the carrier frequency f by values ifs, respectively. However, in case of second and higher harmonic distortion in fs, or due to the modulation process, there will be other side bands or frequencies generated by these harmonics, said side bands having frequencies f-2fs, f+2fs, f f f+3fn, etc., as shown in Figure 18. It has been found that the aforedescribed circuit of Figure 2embodying a sharply tuned im- 7 pedance such as a quartz crystal shunted by a high ohmic resistance (see Figure 9) is sufficiently selective to accentuate anyone of the side bands and at the same time substantially attenuate all the other sidebands in such a manner as to afford a visual observation and study of either the first side band or the higher order side bands obtained in the demodulated signal as will become further apparent from Figure 6.

In the latter there are shown traces (voltag 1) as a function of time) produced on the screen of the oscilloscope representing various wave forms of the detected or output signal if an input carrier signal having a. frequency f modulated in amplitude by an audio. frequency is is impressed upon-the input circuit and if the carrier frequency f is varied slightly so that the various side bands will assume a definite relation to the resonant frequency of the crystal or other high-Q impedance means, so as to accentuate any desired frequency, this being the equivalent of keeping the carrier frequency fixed and varying the 'tuning of the resonant impedance connected to grid l3.

Figure 6A shows the normal curve obtained from the detector by demodulating acarrier .f modulated in accordance with an audio frequency Js, thelatter being represented by the curve in Figure 6A.

Figure 6B shows the demodulation of the second order side band obtained by adjusting the carrier frequency to another value thus indicating a harmonic content of the wave due to distortion of the modulating wave or distortion introduced during the modulation process.

Figure 6C shows thederhodulation of the third order side band obtained by adjusting the car rier frequency to still another value, indicating that a third harmonic component is present in the modulating frequency is or has been introduced during the modulation process.

Figure 6D shows another shape of the fundamental frequency comprising the first sid -band "with a fairly high order of third harmonic content.

Figure 6E shows the relative shape. in output amplitude as a function of change of the resonating frequency\ of the circuit 38-1 using a high-Q tuned circuit vorla. piezo electric crystal provided with either a variable condenser shunted across the crystal or means to vary the pressure applied to the crystal or. the spacing between the crystal holder plates. In this case the input signal, Figure 2, is amplitude modulated and held at a-constant frequency and a suitable time gainers.

base voltage is applied to the horizontal deflect ing plate.

Referring to Figures '7 to 10, there are shown modifications of circuits embodying the basic principle of the invention. In Figure 7 a series tuned path comprising an inductance 50 and a condenser 52, the latter being shunted by a high ohmic resistance 53 to provide a direct current grid return path for the control grid I3, is provided in place of theparallel tuned circuit 30-3! shown in the preceding illustrations. By suitably varying the resonant frequency of this series tuned circuit the phase of the potential established on the grid l3 may be varied from to 180 with respect to the phase of the input potential upon the grid l2 and accordingly in the plate circuit of the tube a variable frequency response will be obtained in substantially the same manner as described hereinbefore. The tube may again serve either as a detector or high frequency amplifier as is understood from the above. In the example illustrated, the tube serves as a detector and for this purpose has a low frequency or audio network inserted in its 5'! to produce a low frequency or audio signal be varied with respect to the phase of the input potential impressed upon .the grid It. At or about the central resonant frequency of the bandpass filter the phase shift will be substantially constant, and at frequencies slightly remote from the center frequency of the filter the phase shift will vary very rapidly. Accordingly, therefore, there will be nearly a constant phase shift of 90 of the potential upon the grid is with respect to the potential upon grid 12 at one discrete band of frequencies, and at frequencies even slightly remote from this band the phase angle between the potentials on the grids l2 and 13 will vary rapidly and consequently a variation of the output response of the circuit will be obtained substantially as shown in Figures 4D and 4E. As will be understood. any other filter or time delay circuit connected to the grid 13 may be employed for vthe purpose of the invention to modify the frequency response characteristic in the manner described.

Referring to Figure 9, there is shown a'system similar to the preceding diagrams embodying a quartz crystal 65 or an equivalent electro-mechanical resonating element connected-between the control grid l3 and ground or cathode. The quartzcrystal is shunted by a high ohmic impedance' 86 to provide a direct current/ return path from the control grid ii to the cathode of the tube. This resistance 66 may be varied to vary the Q of the crystal in order to obtain a resulting in an attenuation or accentuation of broader response curve if desired. The operation of this systemis similar to systems embodying a resonant circuit described hereinabove. Figure 10 shows a circuit similar to.

wherein the single crystaltd has been replaced by a band-pass filter comprising ,three crystals by shunting a pair of further crystals all and 68 connected in' series across the crystal B5 in the manner shown in the drawing. By using three crystals of slightly different frequency, the phase shift of the potential on grid I3 maybe controlled in such a manner as to obtain a fiat top resonance curve with extremely steep sides. In this figure, the tube Hi is assumed to act as a radio frequency amplifier by the provision of a radio frequency transformer 10 in the output circuit,

Figures 11A and 11F show response curves obtained with a resonant impedance means connected to the control grid I3 having an extremely high Q value to enable rejection or accentuation of a single or limited number of frequencies compared with the curves of Figure 4A to Figure 4E based on'substantially lower Q values of the resonant impedance means and resulting in a substantial number. of component frequencies being affected by the selective control action of the grid I3.

Figure 11A shows a normal response curve of an amplifier (output voltage 0 as a function'of the frequency f) with the resonant impedance means according to the invention disconnected from the grid l3. This represents the well known resonance characteristic of a tuned circuit or system for a carrier frequency fo- Figure 113 shows the effect of providing a single high-Q resonant circuit tuned so as to accentuate the carrier frequency and the frequencies on one side of the carrier and to attenuate the frequencies on the other side of the carrier. p

Figure 110 is a similar resonant curve for a two-stage selective amplifier wherein one reso nant impedance provided in one stage according to the invention is tuned to a frequency below the carrier frequency and another resonant impedance associated with the other stagev is Y tuned to a frequency above the oarrierfrequency,

ciate damping reduction system. as shown in Figure 16 to be describe hereafter.

Figure 12 is a vector diagram being further explanatory of the principle and function of the invention. vVectors er and or represent the potentials on the grids l2 and I3, respectively, for

a particular frequency which potentials are at a phase as shown if the resonant impedance connected to grid I3 is in exact tune with the respective input frequency. If the resonant impedance is detuned relative to the input-fre-' quency,-the phase angle of the potential e: rel-' ative to of will become smaller or greater than 90 depending on the sense of detuning as shown at em and er", respectively. The output potential being proportional to the vectorial sumof the potentials er and on will vary accordingly as shown by the vectors E, E and, E", respectively,

the respective frequency which may be a corn- 7 ponent' of a modulationfrequency band. Dee Figure '9 is pending on the Q-value of the resonant imped ance controlling the phase on grid l3, a lesser or greater number of component frequencies will 7 be affected resulting in a' corresponding modification of the resultant frequency response characteristic of the system. In addition to the sum terms in the equation of plate current as a function of the vector potentials on the control grids, the product term will likewise yield a variation in plate current due to the phase shift of the potential on the second grid with respect to th potential impressed upon the first grid. 3

' From the foregoing it will be apparent that it is not necessary for the resonant impedance to be connected'between the grid I3 and cathode, but rather that it is only necessary that this impedance be arranged in a manner so as to change the phase oi. the potential established on the control grid l3 with respect to the input potential impressed upon the grid l2. Accordingly, therefore, Fig. 13 shows a resonant impedance .-means,in the example shown in the form of a quartz crystal, connected between the two icontrol grids l2 and I3. In an arrangement of this type the potential on grid l3 will be in phase with the current through the crystal if the input frequency equals the resonant frequency of the crystal. Above and below the resonant frequency of the crystal the potential developedon grid l3 will be phase shifted. with respect to the input potential on grid 12 as is understood from the above.

If the crystal in Figure 13 is replaced by a resonant circuit, the operation will be in a similar manner. For example, on resonance the phase shift caused by the virtual cathode, ordinarily 90", will be neutralized since the condenser formed by the virtual cathode andthe grid l3- plus the tuning condenser oi the parallel resonant circuit connected between the grids I2 and I! will resonate with the inductance to the desired frequency and accordin ly the phase angle between the r dnotential will be zero and the grids will be effectively cou led by a pure ohmic res stance. Above and below the resonant frequency there will be a varying phase shift between the potent al on grid 13 with respect to the input potential on grid I! in substantially the same manner as described hereinabove.

According to another modification. the functions of he r ds i2 and I 3 may interchanged.

that is. the innut si nal may be impressed upon the grid l3 while the resonant im edance means is connected to the grid l2 and cathode-as sh wn in Figure 14 (parallel tuned circuit 30-3! The operation of this arrangement is substantially the same as has been previously described.

Figure 15 shows a further variation of'the invention wherein the vacuum tube In is provided with three control gridsvizithe input grid l2, a first control grid 13 and an additional control grid Hi, all the three control grids being effectively shielded by a common screen grid 13 connected and biased in the same manner as described above. A first resonant impedance such as parallel tuned circuit 30-3l is connected between the grid l3 and cathode as in the previous circuits and an addition-a1 resonant circuit 30'-3l' is connected between the grid l3 and cathode. By suitably detuning the circuits 30-3l; 30'--3I' above and below, respectively, the frequency of the input signal, a band-pass type filter will be obtained as is understood from the above. The characteristic of such a filter will be as shown in Figures 4D and 4E.

Referring to Figure 16 there is shown a means.

for increasing the Q-value of theparallel tuned circuit 30-3l serving as a resonant impedance connected to the control grid l3. For this purpose, there is shown 'a de-damping circuit or negative resistance in the form of a two-stage resistance coupled amplifier comprising a pair of screen grid amplifier tubes I andfll connected in cascade through a resistance coupling network 11 of known design. The tube has its grid cathode path shunted across the circuit -3l.

network in the cathode-to groundlead for the tube 18. The output of tube 18 is connectedto the input .-of tube 15 through a pair of feedback paths one being of regenerative and the other being of degenerative character and-containing adjustable resistors 8i and 82, respectively, to provide by differential feedback a negative resistmodification of an inventive circuit comprising a pair of electron tubes in place of the single composite tube l0 provided in the preceding diagrams to obtain the results according to the invention. The input signal voltage isimpressed -upon the grid of an ordinary scre'en grid tube 84 including a screen 86 and a plate 81. The resonant impedance such as a parallel tuned circuit 3li3l is connected between the plate 8'! and cathode, that is the plate is operated at substantially cathode potential or at a negative potential with respect to the cathode. Due-to the virtual cathode formed adjacent to the plate 8'! in a manner understood from'the foregoing, the circuit 303i will develop a phase shifted potential onthe plate 81 relative to the input potential on grid 85. In order.- to produce a combined effect of the potentials on the grid 85 and plate 81 a further tube 88 having a pair of control grids 80, 8| which may be separated by a screen grid 92, and having a plate. 93 is provided in the embodiment shown. Control grid 90 is excited by the potential developed-at the plate 81 and control grid 9! is excited by the input potential wherein a radio frequency or audio frequency voltage will be developed in the output circuit of tube 88 of modified frequency characteristic in substantially the same manner as described hereinbefore.-

Figure 19 illustrates a further application of the invention-in the form of a wave analyzer of the harmonic type for determining and studying the harmonic content of electric waves. As has frequency In plus harmonics of both odd and even order.

Item 16 is a degenerative resistance in the cathode lead of this tube and'80 is a biasing,

In other words, fs will be a composite periodic wave containing a fundamental In Figure 19, item 85 is a variable 7 oscillator producing a stable oscillation and being accurately ca1ibrated;' 96 is a buffer amplifier of the band-pass type having a flat frequency response throughout the band over which the oscillating frequency is to be varied; 91 is a modulating amplifier also havinga flat frequency response over the frequency variation range of the oscillator 05. There is further provided an audio frequency amplifier 90 whose gain may be varied and which is flat over the entire audible and supersonic range. 98 represents the input circuit for impressing an unknown audio or supersonic wave whose fundamental frequency and harmonic content it is desired to measure. The output of the modulating amplifier 01 is fed through a suitable attenuator I and hence to the first control grid I2 of valve I0 having a quartz crystal 65 connected to the second control grid I3 in a manner described hereinabove. IN is an audio-frequency filter in the output circuit of tube I0 to develop a voltage impressed upon the vertical deflecting plates I04I04' of a cathode ray oscillograph I02 on the one hand and upon 'a vacuum tube voltmeter I05 on the other hand.

Horizontal deflecting plates I03--I03' of the oscillograph are excited by a suitable linear time base. i a

The operation of the system of Figure '19 is as follows: The amplifier 91 is modulated to a cer-.

tain percentage by. the unknown frequency applied by way of input 98 by suitably varying the gain of the amplifier 99; The oscillator 95 is then set to the frequency of the crystal 65 by observing the vacuum tube volt meter I05. The plate current i of the latter as a function of frequency varies as shown in'Figure when the frequency of the oscillator 95 passes through the frequency of the quartz crystal. The oscillator is set to the central operating point P and the attenuator I00 adjusted so that practically no indication appears on the screen of the oscilloscope as marked in Figure 20. Thenthe oscillator.95 is increased 7 or decreased slowly in frequency to a first point until an audio frequency wave is impressed upon the oscilloscope I02. The vacuum tube voltmeter I05 shows that an audio frequency voltage is impressed at the output of the wave filter. The difference in frequency of the oscillator 95 from its setting at the operating point P to this first point is then the fundamental frequencyjs of the wave being analyzed. Its amplitude may then be set to a predetermined value of say 100% on the vacuum voltmeter by varying the gain of the amplifier 99.

plitude determined. Accordingly, therefore, a complete. analysis of an unknown audio. frequency wave may be readily obtained with this system utilizing the principle of the invention. In an alternative arrangement the oscillator 95 may be fixedly tuned and the crystal 0! may be varied with. respect to the oscillating frequency or an extremely high-Q resonant impedance of variable resonant frequency may be employed in place of the crystal. Similar results are obtained In a like manner the third harmonic may be visually observed and its amcoupling transformer III upon the input of a frequency converter or first detector stage H2 wherein the radio frequency signals are combined with signals of alocal oscillator II3 to produce signals of intermediate frequency impressed upon an intermediate frequency amplifier by way of a transformer II4 fixedly tuned to the intermediate frequency. The intermediate frequency amplifier in the example shown comprises two amplifier stages constructed in a manner proposed second detector including a diode I22 followed by an audio frequency amplifier and a translating device such as a loud speaker in accordance with standard practice. Automatic volume control voltage may be obtained from the diode detector and employed to control the direct current bias on the control gridsof the intermediate fre-' quency amphfier. By replacing the tuned circuits H6 and I20 by quartz crystals, it is possible to obtain a resonance curve with extremely sharp cut-ofis for the receiver. This is particularly useful in communication type receivers where it is desirable to receive a signal in the presence ofan interfering signal only a few cycles away. It has also been found advantageous to make the resonant frequency of-one of the crystals variable over a few kc. and to have-it controlledfrom the front panel of the receiver to allow eflicient single signal reception.

It has been found that by varying the Q of 21' in Figure 1, the sharpness of the response variation may be varied over wide limits. In practical over the desired frequency range say about 20 kc. Once this has been determined the Q of the tuned circuit with the resistor shunted across it is measured and the tunedcircuit is replaced by a circuit having a desired Q value.

Figure 22 illustrates a further application of the invention embodied in a multiplex telegraph system.- A carrier frequency {generated in the transmitter I20 is modulated by a number of modulating frequencies f1, f2, 13 produced by generators I26, I21, I20, etc. At the receiving end a number ofreceivers I30, I32, I33, etc. each inby holding the oscillator at a fixed frequency and by varying this resonant impedance. For example, in actual practice a tuned circuit followed by, an amplifier comprising both degenereluding ajcircuit of the type proposed by the invention'are provided, each including a quartz crystal to serve as selective means for segregating the several modulating frequencies f1, I2, I: and is and applying them to the respective re,-

ceivers operating suitable translating devices such in the preselector stage of a superheterodyne receiver and suitably tuning the resonant impedamplitude of the desired signal frequency. An arrangement of this type is shown in Figure 23 wherein tube I constitutes a radio frequency amplifying stage connected between'the antenna H0 and the frequency converter or mixer I35 of the superheterodyne receiver. By suitably tuning the resonant circuit 30-3i connected to the second control grid of the tube in the manner described and by properly tracking and ganging the tuning condenser 3i with the local oscillator and the remaining radio frequency tuning elements, the image frequency may be rejected substantially within a desired operating range.

It will be evident from the foregoing that the invention is not limited to the specific circuits and arrangements shown and disclosed hereinfor illustration but that the principle and underlying inventive idea are susceptible of numerous variations and modifications coming within the broader scope and spirit of the invention as defined in the appended claims. The specification and drawings are accordingly to'be regarded in an illustrative rather than a limiting sense.

I claim: 1. In a system for translating a modulated signal-wave comprising a carrier and a predetermined band of side frequencies, an input circuit, an electron discharge device comprising a cathode and an anode for producing an electron space current, control means including circuit connec--- tions from said input circuit to said electron device for varying said space current in accordance with said signal wave, an electrostatic control concentrated variable electron space charge addiscrete components of said signal wave of such phase as to modify said input-output frequency response characteristic, and means to provide a high impedance direct current return path from said grid to said cathode.

3. In a system for translating a modulated car- I rier signal wave comprising a carrier and a predetermined band of side frequencies, an input circuit, an electron discharge tube comprising a cathode and anode forproducing an electron space current, control means including circuit connections from said input circuit to said tube for varying said space current in accordance with said signal wave, an electrostatic control electrode in said tube, means for producing a concentrated variable electron space charge adjacent to said control electrode, an output circuit for said tube, the input-output frequency characteristic of said'system having a band width sufficient to pass said carrier and side frequency band, resonant impedance means connected to said control electrode and cathode, said impedance means having a frequency response characteristic of substantially reduced band width compared with said input-output characteristic and having its center frequency tuned to a predetermined side frequency of said signal wave to cause a displacement current to be induced therein by said space output frequency response characteristic.

electrode in said device, means'for producing a jacent to said control electrode, an output circuit for said device; the input-output frequency characteristic of said system having a band width suflicient to pass said carrier and side frequency determined band of side frequencies, an input circuit, an electron discharge tube comprising a cathode and anode for producing an electron space current, control means includingcircuit connections from said input circuit to said tube for varying said space current in accordance with said signal wave, an electrostatic control electrode in said tube, means for producing a concentrated variable electron space charge adjacent to said control electrode, an outputcircuit for said tube, the input-output frequency characteristic of said system having a band width suflicient to pass said carrier and side frequency band, resonant 'impedance means connected to said control electrode and cathode, said impedance means having a frequency response characteristic of substantially'reduced band width compared with said input-output characteristic and havingits center 4. In a system of the character described, an input circuit traversed by a modulated signal current comprising carrier and side band frequencies, an electron discharge tube including means for producing an electron space current, control means including circuit connections from I said circuit to said tube for controlling said space current in accordance with said signal current, an output circuit for said tube, said input and output circuit forming'a translation system having arelatively broadly tuned frequency response characteristic of predetermined band width suflicient to pass said signal current, an electrostatic control electrode in said tube, means for producing a concentrated variable electron space charge the carrier frequency to cause a displacement substantially less than the signal frequency band being translated and being detuned relativeto current to be induced therein by said space charge and to develop a .signal potential upon said control electrode of such phase as to sharpen the frequency cut-off 'of the input-output frequency response characteristic of said system.

' 5. In a system of the characterdescribed, an

input circuit traversed by a modulated signal.

current comprising carrierand said band'frequencies, a piezo-electric crystal element, an electron discharge tube including means for producing an electron space current, control means including circuit connections from said circuit to 2,313,911 band width substantially exceeding the width of connections from said input circuit to said tube for controlling said space current in accordance with the amplitude of the signals being translated, an output circuit for said tube, said system having a relatively broad input-output frequency,

I response characteristic,- an electrostatic control to a predetermined sde band frequency of said signal current and connected to said control electrode and cathode to cause a displacement current to ,be induced therein by said space charge and to develop a signal potential upon said control electrode of such phase as to sharpen the frequency cut-off of the frequency response characteristic of said system.

6. In a system of the character described, an input circuit traversed by a modulated signal current comprising carrier and side band frequencies, an electron discharge tube including means for producing an electron space current, control means including circuit connections from said circuit to said tube for controlling said space current in accordance with said signal current, an output circuit for said tube, said input circuit and output circuit forming a translation system .of predetermined frequency band width sufiicient to pass said signal current, an electrostatic control electrode in said tube, means for producing a concentrated variable electron space charge adjacent said. control electrode, a resonant impedance device having a response band width substantially less than said predetermined band width and being connected to said control electrode and cathode to cause a displacement cur-' rent to be induced therein by said space charge and to develop'a signal potential upon said con-' trol electrode of such phase as to substantially accentuate discrete frequencies of the signal current in said output circuit, and means for varying the relative departure of the center resonant frequency of said impedance device from the carrier frequency of said signal current.

'7. In a system of the character described. an input circuit traversed by a modulated signal current comprising carrier and side band frequencies, a piezo-electric crystal element, an electron discharge tube including means for producing an Y electron space current, control means including circuit connections from said circuit to said tube for controlling saidspace current in accordance with said signal current, an output circuit for said tube, said input and output circuit forming a translation system of predetermined frequency band width substantially exceeding-the width of the frequency response curve of said crystal element and sufficient to pass said signal current, an

, electrostatic control electrode in said tube, means for producing a concentrated variable electron space charge adjacent to said control electrode, said crystal element being connected to said control electrode and cathode to cause adisplacement current to be induced therein by said space charge and to develop a signal potential upon said control electrode of such phase as to substantially accentuate a discrete frequency of the signal current in said output circuit,,and means for varying the relative departure of. the carrier frequency of said signal current from the resonant frequency of said crystal element.

8. In a system for translating modulated signal energy comprising carrier and side band frequencies, an input circuit, an electron discharge tube havingmeans for producing an electron space current,;contro1 means including circuit electrode in said tube, means for producing a concentrated electron space charge adjacent to quency characteristic and being tuned to a predetermined side frequency.

o 9. In a system for translating modulated signal energy comprising carrier and side band frequencies, an input circuit, an electron discharge tube having an anode and a cathode for producing an electron space current, control means including circuit connections from said input circuit to said tube for controlling said space current in accordance with the amplitude of the signals being translated, an output circuit connected to said anode, said system having a relatively broad input-output frequency response characteristic, an electrostatic control grid in said tube, means for producing a concentrated electron space charge adjacent to said control grid, and a direct current impedance-shunted piezo-electric crystal element having a relatively narrow frequency response characteristic connected to said control electrode and cathode and resonating at one of said side band frequencies to modify quency response characteristic.

10. In a system for translating modulated signal energy comprising carrier and side band frequencies, an input and an output circuit, a pair of said freelectron tube amplifiers connected in cascade acteristics connected to each of said control electrodes, one of said impedance means resonating at a frequency above and the other impedance means resonating at a frequency below said-carrier frequency to sharpen the cut-ofis of said quencies, an input and an output circuit, at least two electron tube amplifiers having anode and cathode electrodes and connected in cascade between said input and output circuit to obtain an tubes, means for producinga concentrated electron space charge adjacent to said control grids, a pair of direct current impedance-shunted piezo electric crystal elements each being connected to a one of said control electrodes and the cathode of the respective tube, one of said crystal elements resonating at a first frequency above and the other of said crystal elements resonating at a second frequency below said carrier frequency and symmetrical to said first frequency, to

sharpen the cut-offs of said frequency response v characteristic.

HENRY M. BACH. 

